THE FORMATION OF THE LARVA. 101 



of each pedipalp forms a palp. The cavity of the proboscis leads 

 into a muscular pharynx, which is followed by the cylindrical 

 oesophagus. This latter traverses the central nervous system, which 

 (in Trombidium, Henking) consists of a large ventral ganglionic mass 

 and a pair of smaller supra-oesophageal ganglia. The oesophagus (in 

 Gamasus) passes into the spacious metenteron, from which the 

 hepatic caeca extend anteriorly and posteriorly (Fig. 54, Is). The 

 metenteron narrows again posteriorly and enters the rectal vesicle. 

 Here the two large Malpighian vessels (cm), which have until now 

 been regarded as outgrowths of the proctodaeum, take their rise.* 



If the so-called Malpighian vessels of the Scorpiones and the Araneae should 

 prove to be diverticula of the enteron, as may be conjectured (pp. 20 and 83), 

 their origin would have to be more thoroughly investigated in the Acarina 

 also. Since the proctodaeum in these latter has, as opposed to the enteron, 

 a certain independence (Hexkixg, MacLeod), the question as to the nature 

 of these appendages would perhaps be easier to decide in the Acarina. 



The anus lies at the end of the abdomen, or, as in Trombidium, 

 is shifted forward. In Trombidium there is a constriction in the 

 metenteron between the thorax and the abdomen, and at this point 

 lie (in the first abdominal segment) two bean-shaped bodies which 

 Henking considers to be rudiments of the genital glands. These 

 would therefore at first be paired, and only in the further course 

 of development fuse to form the unpaired genital gland known in 

 the adult. 



Among the internal organs we have still to mention the heart, 

 which is present in some Acarina though not in all. In Gamasus 

 it lies as a rounded organ at the posterior end of the abdomen 

 (Fig. 54, h). It has one pair of ostia and passes anteriorly into an 

 aorta. It is suspended by the fibres of connective tissue or muscle- 

 fibres from the dorsal body-wall. 



The compact form of the heart is connected with the reduction undergone 

 by the whole body in the Acarina. Winkler, who has closely studied this 

 question (No. 105), points out that, in the Pseudoscorpiones (young form of 

 Obisium silvaticum), the heart is still somewhat long, yet is provided with only 

 one pair of ostia (at the posterior end). The heart of the young Phalangid, in 

 which two pairs of ostia occur, is also reduced, though to a less degree. 



In the larva of Trombidium, between the first and second pairs of 

 limbs, there is on each side a crescent-shaped projecting structure 

 (Fig. 52, ut) produced by a thickening of the chitinous cuticle. At 



* [This description is probably correct so far as Gamasus is concerned, except 

 perhaps in the question of where the metenteron ends and the proctodaeum 

 begins, but it must not be applied to all Acarina. — Ed.] 



